Process for electrolytic etching of zirconium and zirconium-base alloys



United States latent Office 3,371,021 Patented Feb. 27, 1968 3,371,021 PROCESS FOR ELECTROLYTIC ETCHHNG (ll? ZIRCONEUM AND ZTRCONIUM-EASE ALLOYS Jacques Delafosse, Gif-sur-Yvette, Jean Herenguel, Versailles, Pierre Leiong, Fontenay-aux-Roses, and Simon Silherman, Cachan, France, assignors to Commissariat a lEnergie Atomique, Paris, France No Drawing. Filed Sept. 22, 1964, Ser. No. 398,423 Claims priority, application France, Oct. 1, 1963,

9 Claims. (Cl. 2tl4--140) A certain number of industrial processes for the etching of zirconium and zirconium-base alloys involve chemical attack in an oxidizing acid mixture having a fluorine ion base. Among these latter, mixtures of nitric acid and hydrofluoric acid with water are the most common.

It is frequently observed during etching of a more or less highly oxidized material that oxide films having a thickness which exceeds a certain value can neither be dissolved nor detached in a satisfactory manner from the base metal. Two cases can accordingly arise:

(at) The oxide film remains on the part, which subsequently has all the adverse effects which are associated with its presence. Those zones which are etched are the seat of selective dissolution processes, thus forming raised portions of surface which are wholly unacceptable in the case of the majority of subsequent applications;

(b) The metal is attacked in an irregular manner beneath the oxide film and, when said film has been completely removed, the surface is raised in a more or less rough and irregular manner, which is also unacceptable in the majority of subsequent applications.

Moreover, when the oxide proves difiicult to attack or to detach, it proves necessary to perform the attacking process for such lengths of time that, when the oxide layer has to be completely removed, the thickness of metal which is consequently removed is unpredictable or incompatible with the dimensional tolerances which have been laid down.

These phenomena are liable to occur in different types of oxide films, especially:

Films which are formed by oxidation in the hot state either in air or any oxidizing atmosphere.

Films which are formed either chemically or electrochemically. For example, in certain electrolytes, the anodic oxide film which is formed prevents the flow of current with the result that, after a very short time, the oxidation rate becomes either very low or zero; such films can be etched only with great difiiculty above a certain thickness.

It has been observed that, if zirconium or one of its alloys is subjected to anodic oxidation in a solution of nitric acid, it is possible to form a film having a thickness ranging from a few microns to several tens of microns in thickness. However, as is well known, oxide is formed only on those portions on which oxide films having the above-mentioned characteristics have not already been formed. In any case, at the end of a certain time of electrolysis, the oxide film breaks away in fragments from the base metal accompanied by crepitation. The period of time after which this phenomenon takes place is a function of the parameters of the electrolysis, namely the concentration of nitric acid, the temperature, current density, etc. but in fact takes place only after a fairly long period of time, that is to say when the thickness of the anodic oxide is fairly substantial.

The present invention has for its object a process for electrolytic etching of zirconium and zirconium-base alloys which is characterized especially by the following features considered either separately or in any operative combination:

(a) The zirconium or zirconium-base :alloys which are placed as anode are subjected to an electrolysis in a mixture of oxidizing acid and fluoride ions under conditions such that an oxide film is developed having a thickness of several microns;

(b) The operation described under (a) can be followed in the same bath by a further electrolysis under a current voltage which is lower than the preceding;

(c) The operation described under (a) can be followed, in a bath having a different concentration of oxidizing acid and of fluoride ions, by a further electrolysis at a voltage which is lower than the preceding;

(d) Operations (a), (b), (c) can be repeated a number of times;

(e) The electrolytic bath is a mixture of nitric acid and hydrofluoric acid which may be diluted if necessary.

The object of the present invention is therefore a method of electrolytic etching of zirconium and zirconiurn-base alloys which makes it possible to remove from the surface of these metals:

(1) Oxidation films which it had not been possible to remove up to the present time by any known means except mechanical means;

(2) Oxidation films which up to the present time could be removed only with difficulty by chemical process and then left a surface which was more or less modified geometrically and which was therefore unfit for many subsequent applications.

In fact, if hydrofluoric acid is added to the standard nitric-acid electrolyte, the following new phenomena appear compared with the known conditions previously referred to:

1) The anodic oxide film of either chemical or electro lytic origin increases underneath any films which may previously have existed, contrary to the process which takes place in the nitric-acid solution when employed alone;

(2) The disintegration and break-away of the anodic oxide film have the effect of carrying away any films which could not previously be polished;

(3) The break-away process takes place in the case of anodic films which are of smaller thickness than in the case of electrochemical etching in a nitric-acid medium, that is to say in the case of films which are formed either at a lower current density or in shorter periods of time;

(4) If the voltage is reduced to a value which is comprised between 0 and 2 volts, the break-away process takes place at an even higher rate, provided that a minimum thickness of anodic oxide has previously been formed in this same bath;

(5) The break-away process which takes place at a fairly slow rate if no voltage is applied to the surface nevertheless takes place at a higher speed if this surface is maintained at an anode potential which is comprised between 0.1 and 2 volts, for example.

In addition, it is observed that, if the oxidation and break-away cycle is repeated a certain number of times, it is possible to remove from the surface of the zirconium or zirconium-base alloys not only films. which would leave an unacceptable surface if chemical etching were performed alone, but also films which it was not hitherto possible to remove by any means other than mechanical means (grinding, scraping, sand-blasting and so forth).

The electrolytic etching process leaves a smooth surface and makes it possible to maintain geometrical dimensions within predictable tolerance ranges.

One preferred form of embodiment of the invention consists in placing the parts to be etched as anode in an electrolytic bath containing from 7 to 70 volumes of nitric acid at 42 B., from 0.2 to 10 volumes of 40% hydrofluoric acid, the remainder being water, and in subjecting said parts at a temperature comprised between and C. to the following operations:

(a) Formation of an anodic film on the surface with a current density of 0.7 to 4 arnps/dm. for a period of 2 to minutes;

(b) Break-away of the anodic film which carries away simultaneously those oxide films which were initially present, at a voltage of 0.1 to 3 volts for a period of 10 seconds to 20 minutes.

The succession of two cycles can be repeated as many times as may be desired; the thickness of metal trans formed into oxide is approximately 0.436 per amp/dm. and per minute of the first oxidation stage.

There will now be described below a number of different examples which are given without implied limitation and which relate to the practical application of the process .for electrolytic etching of zirconium and zirconium-base alloys in accordance with the invention. The practical arrangements which will be described in connection with these examples must be considered as forming .part of the invention, it being understood that any and all equivalent arrangements could equally well be employed without thereby departing from the scope of this invention.

Example I A zirconium part which has been heated in air at 570 C. for a period of 5 minutes cannot be polished by chemical process in conventional baths such as, for example, a bath made up as follows:

Percent by vol.

Nitric acid at 42 B. 45 Hydrofluoric acid of concentration 5 Water 50 On the other hand, the same part is etched electrolytically under the following conditions:

At the end of 20 to 30 seconds, the film begins to break away from the part in fine particles which fall into the bottom of the tank. The complete break-away process lasts from 15 seconds to one minute. After withdrawal from the bath, the part is found to have a smooth surface.

Example II A part formed of Zircaloy 2 which has been heated in air at 570 C. for a period of 5 minutes cannot be polished by chemical process in any conventional bath such as the following:

Percent by vol.

Nitric acid at 42 B 45 Hydrofluoric acid of 40% concentration 5 Water M On the other hand, the same part is etched electrolytically under the following conditions:

Nitric acid at 40 B Percent by vol 50 Hydrofluoric acid of 40% concentration do 1.2 Water do 48.8 Temperature C 20 and in the following successive stages.

4 1st stage:

Current density amps/dm. 3 Time'duration minutes 10 The voltage is increased from 2 to 8 volts.

2nd stage: voltage-4 volt.

At the end of 20 to 30 seconds, the film begins to break away from the part in fine particles which fall to the bottom of the tank. The complete break-away process lasts from 15 seconds to one minute. After withdrawal from the bath, the surface of the part is found to be smooth.

Example Ill An assembly formed of Zircaloy 2 was constructed by argon-shielded arc-welding process but with insufiicient shielding of the surfaces. The result thus achieved was the formation of a thermal oxide film on the weld bead and in the areas adjacent thereto.

A first sample was processed in the chemical etching bath mentioned in Example I so as to remove 50 microns per face. The film was removed in an irregular manner and left local surface roughness.

By way of comparison, a second sample was etched by electrolytic process under the following conditions.

Composition of the electrolyte:

Nitric acid at 42 B Percent by vol 40 Hydrofiuoric acid of 40% concentration do 1.5 Water do 58.5 Temperature C 20 1st stage:

Current density "amps/din?" 3.5 Time-duration minutes 8 The voltage is increased from 2.5 volts to 7 volts. 2nd stage: voltage-1.5 volts.

After a period of one minute, the anodic film breaks away and carries with it the thermal film. Break-away is complete in 15 seconds. The part has lost 10 microns per face approximately and can then be polished by chemical process for the purpose of removing the 50 microns per face prior to carrying out the corrosion resistance test.

The three samples mentioned above were subjected to the conventional corrosion resistance test employed in the case of zirconium and zirconium-base alloys by exposing said samples to steam at 400 C. at a pressure of bars over a period of three days. In the case of the sample which had been subjected only to the chemical etching process, it was observed that the weld bead and adjacent areas were covered with a white powdery film. This film is due to the presence of thermal oxide and above all to the subjacent zone whichhad been contaminated (by heating in the presence of air) and which had not been completely dissolved as a result of the barrier effect of the thermal oxide film. The weight gain calculated in respect of the total surface area of the sample was 57.5 mg./dm. On the other hand, in the case of the sample which had previously been etched by electrolytic process, this sample had a practically uniform, black appearance, that is to say normally satisfactory for this type of exposure, the welding zone being little different from the remainder. The weight gain of this sample was 27.6 mg./dm.

The same results were obtained in the case of unalloyed zirconium and ordinary industrial zirconium-base alloys.

What we claim is:

1. A process for the electrolytic etching of a workpiece composed of zirconium or zirconium alloys which comprises:

(a) positioning said workpiece as the anode in an electrolytic system comprising a cathode, an anode and an aqueous electrolytic bath consisting of a mixture of nitric acid and hydrofluoric acid;

(b) impressing a potential across said electrodes sufficient to cause an electric current having a density of from 0.7 to 4 amps/elm. to flow between said electrodes for a time sufficient to form an oxide film having a thickness of more than 6 microns on said workpiece, and then (c) impressing across said electrodes a potential of form 0.1 to 3 volts for a time sufficient to remove said oxide film.

2. A process in accordance with claim 1, wherein the electrolytic bath consists of a mixture which contains between 7 and 70 volumes of nitric acid at 42 B., and between 0.2 and 10 volumes of 40% hydrofluoric acid, the remainder being water.

3. A process in accordance with claim 1, wherein the total processing time ranges from 2 to 30 minutes.

4. A process in accordance with claim 1, wherein the temperature of the electrolytic bath is from 10 to 25 C.

5. A process in accordance with claim 1, wherein step (b) is carried out until the oxide film reaches a thickness of the order of 10 microns.

6. A process in accordance with claim 1, wherein step 6 (c) is carried out at a voltage which is lower than that of step (b).

7. A process in accordance with claim 6, wherein the voltage used in step (c) is maintained for a period of time of from 10 seconds to 20 minutes.

8. A process in accordance with claim 1, wherein step (c) is carried out at a voltage which is lower than that used in step (b) and in an electrolytic bath having concentrations of nitric acid and of hydrofluoric acid different from those concentrations used in the bath as recited in step (a).

9. A process in accordance with claim 1, wherein the entire process is repeated at least two times.

References Cited UNITED STATES PATENTS 2,780,594 2/ 1957 Dailey 204144 3,006,827 10/1961 Capuano 204141 3,242,062 3/1966 Covington 204-143 HOWARD S. WILLIAMS, Primary Examiner. JOHN H. MACK, ROBERT K. MIHALEK, Examiners. 

1. A PROCESS FOR THE ELECTROLYTIC ETCHING OF A WORKPIECE COMPOSED OF ZIRCONIUM OR ZIRCONIUM ALLOYS WHICH COMPRISES: (A) POSITIONING SAID WORKPIECE A STHE ANODE IN AN ELECTROLYTIC SYSTEM COMPRISING A CATHODE, AN ANODE AND AN AQUEOUS ELECTROLYTIC BATH CONSISTING OF A MIXTURE OF NITRIC ACID AND HYDROFLUORIC ACID; (B) IMPRESSING A POTENTIAL ACROSS SAID ELECTRODES SUFFICIENT TO CAUSE AN ELECTRIC CURENT HAVING A DENSITY OF FROM 0.7 TO 4 AMPS/DM.2 T FLOW BETWEEN SAID ELECTRODES FOR A TME SUFFICIENT TO FORM AN OXIDE FILM HAVING A THICKNESS OF MORE THAN 6 MICRONS ON SAID WORKPIECE, AND THEN (C) IMPRESSING ACROSS SAID ELECTRODES A POTENTIAL OF FORM 3.1 TO 3 VOLTS FOR A TIME SUFFICIENT TO REMOVE SAID OXIDE FILM. 